Method to estimate the basal turn length in inner ear malformation types

The mathematical equations to estimate cochlear duct length (CDL) using cochlear parameters such as basal turn diameter (A-value) and width (B-value) are currently applied for cochleae with two and a half turns of normal development. Most of the inner ear malformation (IEM) types have either  less than two and a half cochlear turns or have a cystic apex, making the current available CDL equations unsuitable for cochleae with abnormal anatomies. Therefore, this study aimed to estimate the basal turn length (BTL) from the cochlear parameters of different anatomical types, including normal anatomy; enlarged vestibular aqueduct; incomplete partition types I, II, and III; and cochlear hypoplasia. The lateral wall was manually tracked for 360° of the angular depth, along with the A and B values in the oblique coronal view for all anatomical types. A strong positive linear correlation was observed between BTL and the A- (r2 = 0.74) and B-values (r2 = 0.84). The multiple linear regression model to predict the BTL from the A-and B-values resulted in the following equation (estimated BTL = [A × 1.04] + [B × 1.89] − 0.92). The manually measured and estimated BTL differed by 1.12%. The proposed equation could be beneficial in adequately selecting an electrode that covers the basal turn in deformed cochleae.

www.nature.com/scientificreports/ compartments 10,11 . More electrical coverage can be provided in IP II and EVAS types depending on how far the cochlear lumen is further developed before the cystic apex and the number of turns available in the cochlear hypoplasia (CH) type with a clinical implication of selecting an appropriate electrode array length. This raised our interest in formulating a mathematical equation to estimate the BTL in IEM types by applying the basal turn diameter (A value) and width (B value), as described by Escude et al. 1 .

Methods
Image analyses. The CT scans from 2013 until the end of 2021 were considered for analysis. Based on the radiologist's report, CT scans with cochlear aplasia, ossified cochleae, temporal bone fracture, and common cavity were excluded. Inclusion criteria included CT scans with other cochlear malformation types, including normal anatomy (NA) in any age group and gender. The CT images were analyzed using 3D slicer software, version 4.10.2, freeware (https:// www. slicer. org/). The A-value was measured in cochlear view starting at the entrance of the round window (RW) and passing through the mid-modiolar section to the opposite side of the LW in "cochlear view" as described previously by Escude et al. 1 . The B value was measured perpendicular to the A value in the cochlear view, which measured the longest distance. The BTL was tracked manually using small segments following the LW in the oblique coronal view of the cochlear basal turn from the RW to an angular depth of 360°. Figure 1 shows A-and B-values, and BTL manually measured in the cochlear view. Informed consent. The informed consent was waived off due to retrospective nature of the study by the same local committee.

Results
Data analyses. Of the 95 CT scans investigated, IEM types including EVAS with better cochlear formation were identified in 24, IP type I in 22, IP type II with EVA in 12, IP type III in 12, and CH type in 14 CT scans. The CT scans with NA type from the last 11 consecutive CI surgeries from our center were used as controls to compare the cochlear dimensions between the IEM and NA types. Within the CH type, the basal turn appeared normal, and with further development of the cochlear turns in five CT scans, leaving the other nine CT scans only with the basal turn. Figure Fig. 3. The A-values of the NA compared to those of the cochleae with IP types I (p = 0.003), II (p = 0.02), III (p < 0.001), and CH (p < 0.01) were significantly different. IP types I (p = 0.01) and II (p = 0.03) were also significantly different from IP type III in terms of A-value. IP type III was not significantly different from CH type (p = 0.36). The B-values of NA compared to the cochleae with IP types I (P < 0.001), II (P = 0.03), III (P < 0.001), and CH (P < 0.0001) were significantly different. IP type I (p = 0.1) and CH (p = 0.4) were not significantly different from IP type III in terms of B-value. The BTL of the NA cochleae was significantly different from that of the cochleae with IP types I (p < 0.01), II (p < 0.05), III (p < 0.001), and CH (p < 0.01). The BTL of IP type I was not significantly different from that of IP type II (P = 0.054). The BTL of IP type III was significantly different from that of IP type II (p < 0.01), but not with IP type I (p = 0.054).    Escude's estimation of BTL was on average 1.03 mm for NA, 1.55 mm for EVAS, and 1.91 mm, 0.97 mm, and 1.89 mm for IP types I, II, and III, respectively higher in comparison to our estimation.

Discussion
Estimation of the CDL using mathematical equations for various angular insertion depths by applying the Aand/or B-values as the input has been validated for NA cochleae 2-4 . These mathematical equations have been included in clinically approved otological pre-planning software tools for estimating the cochlear size and choosing appropriate electrode lengths 5 . However, the IEM types are still not completely addressed when estimating cochlear length. Therefore, this study attempted to find a clinically feasible solution to estimate the BTL in IEM types with A and B values as input.   www.nature.com/scientificreports/ The LW can be manually tracked/measured for an angular depth of 360° from clinical CT images in the oblique coronal view, and this has been reported in NA cochleae by Adunka et al. 12 . This motivated us to manually measure the BTL (360°) following the LW from 95 datasets, including IEM types of EVAS; IP types I, II, III; and CH, and compare it with the NA cochleae. While it is feasible to measure the BTL manually from clinical CT, the BTL can be estimated from cochlear parameters such as A-and B-values.
Plotting the A and B values against the BTLs of all anatomical types showed a strong positive linear correlation with the overlapping of data points from all six anatomical types studied. This implies that the BTL (360° coverage) of all these anatomical types can be estimated from basic cochlear parameters, which is a novel finding of this study. From our clinical experience, we learned that placing a longer length electrode array (31 mm and 28 mm) beyond 360° of angular depth in IP type I resulted in tip fold-over, as the cochlear apex in IP type I beyond the 360° mark is cystic 13 . This was another reason that motivated us to look for a method to estimate BTL to limit electrode placement to 360° of angular depth in difficult anatomies.
In comparison to Escude's mathematical equation involving the A-value alone as input, our estimation of BTL involves both A-and B-values as inputs, minimizing the error between the measured and estimated BTLs. Khurayzi et al. 14 reported the importance of the B-value along with the A-value in defining the shape of the cochlear basal turn, which was reflected in minimizing the error in our study.
Considering only the eight CT image datasets that were used in the manual measurement of BTL in NA cochleae by Adunka et al., the number of CT image datasets used in the current study was relatively high. This increases the confidence level of the mathematical equation formulated for the estimation of BTL. The LW was observed extending beyond 360° in EVAS, IP type II, and in some samples of CH, which needs to be studied systematically in the future.

Conclusion
The LW of IP types I and III can be tracked consistently to 360° of angular depth in the cochlear view, whereas it extends more than 360° in IP type II, EVAS, and CH types. The finding of a positive linear correlation between BTL and cochlear parameters supports the estimation of BTL using the basic cochlear parameters of A and B values in the IEM types. The mathematical equation proposed in this study could be helpful in choosing electrode array length covering the basal turn in malformed cochleae.